Service Manuals, User Guides, Schematic Diagrams or docs for : Keithley 168 168(Model168_Autoranging)
<< Back | HomeMost service manuals and schematics are PDF files, so You will need Adobre Acrobat Reader to view : Acrobat Download Some of the files are DjVu format. Readers and resources available here : DjVu Resources
For the compressed files, most common are zip and rar. Please, extract files with Your favorite compression software ( WinZip, WinRAR ... ) before viewing. If a document has multiple parts, You should download all, before extracting.
Good luck. Repair on Your own risk. Make sure You know what You are doing.
Image preview - the first page of the document
>> Download 168(Model168_Autoranging) documenatation <<
Text preview - extract from the document
INSTRUCTION MANUAL
MODEL 168
AUTORANGING DMM
KEITHLEY INSTRUMENTS. INC
CONTENTS
`rifle page ............................ i
content3 ............................. ii
List Of illuarrarions. ...................... iii
Dimensional "ata ......................... iv
Specificarions .......................... y
1. General Informutio,, ......................
I-I. T,"tr"duction ...................... I
1-2. Warranty Inf"rmation .................. 1
1-3. Change Notice. .....................
2. Initial Prepururian. ..................... :
2-1. General. ........................ 2
L-2. Inspfciion .......................
1-3. Preparation for "xe. .................. 5
3. operating InSrruCLionS .................... 12
3-1. General. ... ;;
1-2. HOW Select Power.
to .......................................
3-3. HOW Make Input Cannccrions. .............
f" 1~3
3-4. ii"W to Select Funcri~o" ................. ;;
1-5. HOW Measure "olttlge .................
to
3-6. HUWLOMeaSureCurrent ................. 19
3-7. How 1" Measure Resistance. ............... 26
4. n,eol;y Of operation. ..................... :;
4-1. General. ........................
4-2. halOg circuitry .................... 31
4-1. DC Voltage Operario" .................. 34
4-4. nc "oltvge operation .................. 16
4-5. "HMS OperaLion ..................... 38
4-6. current Operation. ;;
4-7. Power supply .........................................
4-8. Digital circuitry. ................... 4,
5. MainLenance. ......................... 50
5-1. General. ........................ 50
5-2. Required Test Equipment. ................ 50
5-3. Performance "eliflcarlon ................
5-4. AdjusrmentiCalibrarion Procedure ............ ::
5-5. 'rrouhleshooting........................................... ;;
6. Replacevble Partr.
6-1. General. ........................ 71
6-2. Ordering IniormziLion .................. 72
6-3. cross Reference. .................... 73
6-4. PariS List 74
7. Schematic Diagrams ...................... 85
I MODEL 168 ILLUSTRATIONS
ILLUSTRATIONS
F
LOClfi"" Of current Fan
M,
I SPBCIFIcATIONS MODEL 168
ml-
12.75
(324)
:i
DIMENSIONS IN INCHES (MM)
FIGURE 1. Dimensional m.ta.
iv 0374
MODEL 168
SPECIFICATIONS
I SPECIFICATl"NS MODEL
168
FIGURE 2. n-ant Panel
0374
I MODEL 168 GENERAL INFORMATION
SECTION 1. GENERAL INFORMATION
l-1. INTRODUCTION. The Model 168 is a versatile autoranging digital mul-
timeter useful for meaeurement of ac and dc voltage, ac and dc curre"t.
and resistance. Voltage meaS"remenfs can be made from f.OOO1 volt to t1000
volts dc or .OOOlvolr to 500 volts ac. current meaSureme"es can be made
from .OOOl milliampere to 1 ampere BC and dc in two spans. RCS3fSta"CEY
measurements can be made from 0.1 ohm to 20 me&m in two overlapping
spans. Range and polarity is automatically selected. In addition to the
display of digits, the 168 indicates decimal point, function (AC or DC),
and measurement unit (IDA, A, kn, MR).
1-2. WARRANTYINFORMATION. The Warranty is give" on the inside front
cover of this Instruction Manual. If there is a need to exercise the
Warranty, contacf the Keiehley Representative I" your area to de~emine
the proper action co be r&en. Keirhley maintains service facilities in
England, west Germany, as we11 as in the United states. Check the inside
front cover of this Instruction Manual far addresses.
1-3. CHANGE NOTICBS. 1mprovemenrs or changes to the instrument which
occur after printing of the Instruction Manual will be explained on a
Change Notice sheet attached to the inside back cover.
I 0374 1
`i
,,
CONDENSEDOPERATING INSTRUCTIONS
(N&M, 11'7)
LINE SWITCHES SET FOR
105 TO 125 VAC OPERATION
4 0374
SECTION 2. INITIAL PREPARATION
2-l. GENERAL. This section provides information needed for incoming in-
spection and preparation for use.
2-3. PREPARATION FOR "SE. The Model 168 is shipped ready-to-use. The in-
strument may be powered from line voltage or from rechargeable nickel-cad-
mium batteries (when the optional Model 1688 Rechargeable Battery Set is
installed).
a. HOW to operate From Line Power. The Model 168 provides a three-
wire line cord which mates with third-wire grounded receptacles. The per-
manently installed line cord is stored by wrapping the cord around the
base of the instrument aa shown in Figure 4.
1. liow to Set Line Switches. The Model 168 has two rear panel line
switches which are used to select line voltage ranges of 90-llOV, 105-
125", 195-235", or 210-ZSOV. The line swirches are idenrified as 117f
,234V (5102) and LOW/NORM (5101). Once rhe line voltage to be used has
been defermined, then the line voltage range should be selected from
the four ranges available on the Model 168. For example, when the line
voltage to be used is within the range from 105 fo 125 volts, then ihe
line switches should be set to "117V" and "NORM" ,msitia"s. If the line
voltage to be used is within either of two overlapping ranges, such as
107 volte, then either range may be selected (117V, LOW or 117V, NORM,
for this pareicu1ar example). Line voltages which are not covered by
anyone of the four ranges are not useable. After the line voltsw
switches are set, connecf rhe line cord and depress the LINE pushbutton
to operate.
2. Line Fuse Requirements. The Model 168 requires f"o line fuses
co protect the line-operated power supply. The fuse types are l/8
ampere, 3AG slo-blo. Replacement instructions are given in section 5.
0374 5
I INITIAL PREPAiUTION MODEL 168
FIGURE 6. Model 1688 Rechargeable Battery Pack.
0374
I MODEL 168 INITIAL PREPARATION
b. HOW to operate From Battery .A Power (Model 1688 Rechargeable
Battery set). The Model 168 may be operated from rechargeable nickel-
cadmium batteries when ehe optional Model 1688 Rechargeable Battery Set
is installed. The Model 1688 may be field installed at any time or may
be ordered factory installed. The Rechargeable Battery Set includes a
battery pack which mounts within rhe Modal 168. Wiring is accomplished
by a single plug-i" connector. Battery operation from fully-charged
NI-CAD hareeries is eypically 6 hours.
1. How to Install Model 1688 Rechargeable Bateery Set. The bae-
teriea furaiahed with the Model 1688 come already fnstal~led in the
battery pack. The battery pack includes 7 rechargeable "C" cells
(LZV, 2 AMP HR) and 2 rechargeable "button" cella (8.4V, .225 AMP
HR). If batteries need ea be replaced or re-installed, be certain
to observe the proper polarity of individual cells as shown in
Figure 6. TO insrall the Model 1688 Battery Pack, turn the instru-
ment over 80 that the bottom co"er faces up. Remove four slotted
BCT~WB on the bottom cover as shown in Figure 4. (A chisel-blade
screwdriver is required to loosen the sloteed screws.) Turn o"er
the instrument with COP co"er facing up, taking,care to hold the top
and bottom covers together. Remove the top co"er to gain access to
the printed circuit board. Check to see ebat the four insulating
standoffs are in position on the printed circuit board. Place the
Model 1688 Battery Pack in position an the standoffs with the cable
oriented 88 shown in Figure 8. Plug the 4-wire connector (5103) in-
to the mating receptacle 0'103) taking care to oriene the connector
as shown in Figure 8. After the Battery Pack is inscalled, replace
the top cover. Turn o"er the instrument with bottom cover facing up
and inscall the four slatted-head screws.
TABLE 2-l.
Summary of Batteries Used in Model 1688.
Quanticv Keithley Part No.
I I
Rechargeable "C" cell, 7 BA-30
1.2". 4 Am-km
Rechargeable "Button" type 2 BA-29
battery, 8.4V, .225 AMP-
HR (4 individual 1.2V
Cells)
0374 7
I INITIAL PREPAR4TION MODEL 168
s
h
FICUKE 7. Exploded View of Model 168 with Model 1688.
0374
MODEL 168 INITIAL PREPARATION
MODEL 1688 BATTERY PACK
MODEL 168 -".--_-
0374 9
INITIAL PREPARATION
MODEL 168
FIGURE 9. Bath-~ `feet ~ocatian.
10
0374
I MODEL 168 INITIAL PREPARATION
2. How to Check Batteries. (Valid only in BAT mode). The Model 168
provides two fest points (A and 8) located on the bottom of the instru-
ment as shown in Figure 9. These test points permit a convenient check
of the condition of the internal Battery Pack without need to remove
the Model 168 cover. The voltage at test points A or B may be meas-
ured using the Model 168 or any other comparable voltage measuring
instrument. To check the voltages at test points A or B, select the
BAT mode, connect rhe HI terminal of the Model 168 (for a self check)
or oeher voltmeter to fest points A or B and observe the measured volt-
age. (If a separate voltmeter is used, it is necessary to make a con-
nection to the LO terminal of the Model 168 since both points A and B
are to be referenced to circuit low.) Table 2-2 gives the battery
voltages required at each teat point.
The instrument mutt be operated in the BAT made in order to obtain
a valid battery condition at test points A and B. This will ensure
that the batteries are supplying power to the instrument. If the
voltages are measured when the Model 168 is operated in the LINE
made a different reading may be observed since the batteries are
not connected and therefore do not supply power to the instrument.
TABLE 2-2.
Summary of Battery Voltage Levels (BAT mode)
Test Point Acceprable Battery Levels Recharge Batteries
&+*g,e Normal if below Tested _
A 2.9 - 9" 4.8V 2.w B&29
B 7v - 10.5" 8.4V 7v RA-30
3. HOW fo Charge Batteries. The Model 168 provides built-i" re-
charging circuitry for recharging the Model 1688 Battery Pack. To re-
charge the internal batteries, connect the Model 168 to line power and
depress the CHG pushbutton. Recharging time is dependent on the condi-
tion of the batteries at the time of recharge. Typically, the recharge
time is l-112 hours per bow of discharge (or 9 hours of charging time
for every 6 hours of operating time in the battery mode).
NOTE
The Model 168 may be operated while in the CHG mode. However,
if the Battery Pack is not installed, the Model 168 will not be
operable when the CHG mode is selected since the batteries are
connected in series with the line power supply.
0374 11
-
OPERATING INSTR"CTIONS 'MODEL 168
SECTION 3. OPERATING INSTRUCTIONS
3-l. GENERAL. This section provides infarmation needed to operate the
Model 168 for measurmene of voltage, current, and resistance.
3-2. HOW TO SELECT POWER. The Model 168 may be powered from line voltage
or rechargeable nickel-cadmium bafteries (when the Model 1688 is installed).
The Model 168 has a built-in line-voltage power supply and line cord. If
the accessory Model 1688 rechargeable Battery Set is ordered and installed,
then Lhe user has the option of selecting line or battery operation via the
front panel pusbbuteons labeled LINE and BAT.
NOTE
The accessory Model 1688 Rechargeable Battery Set my be ordered at
the fime of purchase of the Model 168 or may be purchased and field
installed at a later time if so desired. The Model 1688 features
plug-in wiring. As a result, no modifications need to be made to
the Model 168 chassis.
a. HO" to operate From Line Power. The Model 168 is operable over four
ranges of line voltage from a minimum of 90 volts to a maximum of 250 volts,
ms, 50-60 Hz. me line voltage ranges are 90-llov, 105-125V, 195-235V, and
ZlO-25OV.
TABLE 3-l.
summary of Line Voltage Switch settinga
sear Panel Line Voltage Ranges - Volts, nns, 50-60Hz
Line S"fGxhf?S 90-110 105-125 195-235 210-250
117/234V Switch (5102) 117 117 234 234
LOW/NORMSwitch (SlOl) LOW NORM LO" NORM
b. HO" to operate From Battery Power. The Model 168 my be used with
the optional accessory Model 1688 Rechargeable Battery Set to provide
portable operation in addition to line operation. To operate from battery
power, depress the BAT pushbutton. Check the battery vaJtage at test
points A and B co ensure that barreries are charged sufficiently.
12 0374
MODEL 168 OPERATING INSTRUCTIONS
Refer to Section 2-3b for instructions concerning installation of the Bat-
eery Pack, battery voltage checks, and recharging. Battery operation from
fully charged NI-CAD batteries is 6 hours minimum. No fuses are required
for operaeion in battery mode.
TABLE 3-2.
Summary of flattery Voltage Levels (BAT mode)
Test Point Acceptable Battery Levels Recharge Bdtteries
RtS",P$ Normal if below Tested
A 2.5V - 9V 4.m 2.5V M-29
8 7v - 10.5v 8.4" 7V b-30
TABLE 3-3.
Summary of Operation in LINE, CHG, and BAT Modes.
Condition of Inserument:
PUShbUttOn Line connected Line connected Line not-connected
Depressed 1688 not installed 1688 installed 1680 installed
OFF OFF OFF OFF
LINE ON ON OFF
CHG OFF ON OFF
BAT OFF ON ON
NOTE
The insfrument will be turned off if all power pushbuttons are
released (all non-depressed). A lock-out feature prohibits
selection of two or more puehbuttons at the same time.
3-3. HOWTO MAKE INPUT CONNECTIONS. The Model 168 has two front panel
terminals identified aa "HI" (red) and "LO" (black). These terminals
accomodate banana plugs, alligator clips, spade lugs, bare wires, and
other similar input connections. Leads may be fabricated using a good
quality copper wire terminated by single banana plugs such as Keithley
Part No. 80-S or dual banana plug such as Keitbley Part No. W-7.
Rraay-made tese lead8 are also available from Keifhley. accessory Model
1681 Clip-on Test Lead Set includes two 40 inch long leads terminated by
a banana plug and spring-loaded clip which easily attaches to wires and
terminals on pc boards, etc. Model 1683 Universal Test Lead Kit features
interchangeable probe tips for various applications. The Kit includes
regular probes, alligator clipe, banana plugs. spade lugs, and phone tips.
0374 13
OPERATING INSTR"CTIONS MODEL 168
14 0374
3-4. HOW TO SELECT FUNCTION. The front panel pushbuttons are arranged
to permit selection of five functions including DC voltage, AC voltage,
DC curcent, AC current, and resistance.
a. DC Voltage. To select DC voltage operation, depress the "DC"
pushbutton. Voltage function is implied by the "DC" function indicator.
b. AC Voltage. To aelect AC voltage operation, depress the "AC" push-
button. Voltage function is implied by the "AC" function indicator.
C. DC current. To select DC current operation, depress the "DC" push-
button. Then select either the "ti" or "A" pushbutton depending on the de-
sired range. The %A" and "A" pushbuttons are "push-push" type with lock-out,
such that one pushbutton muet he released before the other can be selected.
d. AC Current. To select AC current operation, depress the "AC" push-
button. Then select either the "mA" or `A" pushbutton depending on the
desired currenf range. The "mA" and `A" pushbuetons are "push-push"
type with lock-out, such that one pushbutton must be releaeed before the
other can be selected.
e. Re8iStance. To select resistance operation, depress either the
"LO OHMS" or "HI OHMS" pushbutton depending an the desired resistance
range. Either "kR" or "Mn" are displayed depending on range.
NOTE
If all function pushbuttons are released, the inpue to the Model
168 will be disconnected. A lock-out feature prohibits selection
of inconsistent funcrlon pushburtons at the same time.
TABLE 3-4.
Summary of Function Settings
Function Pushbuttons Depressed
Des lred Voltage & Current CUrrent Resistance
DC AC d A LO OHMS "I OHM
DC Voltage x
DC Milliamperes x x
DC Amperes x x
AC "oltage x
AC Milliamperes x x
AC Amperes x x
Resistance (LO) x
Resistance (HI) x
OPERATING INSTRUCTIONS MODEL 168
3-5. HOW TO MEASURE VOLTAGE. The Model 168 measures ac and dc voltage
in five ranges: O.lV, lv, IO", IOOV, and IOOOV dc orlOOOVac (both ac
and dc volts have a maximum inpur of 1200 volts peak ac + dc). The
displayed voltage is direct-reading with decimal point located auto-
nlatica11y.
a. DC Voltage. The Model 168 automatically displays voltage from
f.OOO1 valts dc to $999 voles dc. Above 999 volts, the display reads
properly and flashes to indicate an overrange condition. If the polarity
at the HI eerminal is negative, the Model 168 display indicates a (-)
minus sign. If the minus sign is off, a positive voltage is implied. A
lighted "DC" and decimal point are also displayed for all dc measurements.
1. How to Measure DC Voltage. Select the dc function by depressing
the "DC" pushbutton. Connect the signal to be measured between I+1 and
I.0 terminals. Observe the displayed digits, polarity sign, and decimal
point lacacion. The lighted "DC" indicates that the dc voltage func-
tion has been selected.
2. How to Select Range. The Model 168 autamaeically selects the
appropriare range for all valtage measurements.
3. How to Determin+ Accuracy. The Model 168 accuracy is ~0.1% of
reading ~1 digit. For example, a display reading of 1.000 volts dc
will have an uncertainry of ~0.1% 51 digit or +.002 volts. The input
resistance in rhe dc mode is 10 megohms. Measurements from relatively
high source resistances could cause an additional reading error. The
amount of error due to loading can be determined by ehe following
relatianship:
% error = 100 x Ks i (Es i 107)
where R, = source resistance in ohms.
For example, a scurce resistance of 10,000 ohms will result in a
loading error of approximaeely 0.1% of reading.
4. How fca Determine Maximum Allowable Input. The maximum allowable
voltage input is 1200 volts dc + peak ac. The Model 168 displays dc
voltages to i999 volts. Beyond 999 volts fhe display reads properly
and blinks to indicafe an overrange condition.
IMPORTANT
The Model 168 provides ac rejection of greater than 75 dB (NMRx).
However, a large ac signal superimposed on a dc level could case
damage if the inpue exceeds 1200 volts dc + peak ac.
16 0374
/
MODEL 168 "PERATINC TNSTRUCTTONS
5. HOW to ZerO the Display. The Model 168 has a front panel zero ad-
juscment which may he used to zero the display to compensate for zero
offset. Apply a &xc connection bctwee" the input terminals or select
"A" function and adjust the zero control (screwdriver required) to ob-
tain a .OOOO display with the minus (-) polarity flashing on and off.
b. AC Voltage. The Model 168 automatically displays voltage fror .OOO1
volts `co 499 volts ac rms. Above 499 volts, the display reads properly
and flashes to indicate an overrange condition. In the ac function, the
Model 168 operates as an average-reading voltmeter, calibrated in terms of
the root-mean-square (rms) of a sine wave. `The ac-to-dc converter is a full-
wa"e rectifier type, and as such the calibration is exact for sinusoidal
waveforms. The input signal is ac coupled (capacitive) to the input amp-
lifier so that dc is blocked. me 1np"t bLocking capacitor effectively
reduces the Model 168 low-frequency response to approximately 20 HZ.
1. HOW to Measure AC Voltage. Select the ac function by depressing
the "AC" pushbutton. Connect the signal to be measured between HI and
LO terminals. Observe the displayed digits, and decimal point location.
The lighted "AC" indicates chat the ac voltage function has been selected.
2. mw to se1ecr Range. `The Model 168 auramarically selects tire
appropriate range for a volksge Ineas"reme"t.
3. How to Determine Accuracy. The Model 168 accuracy is 1(0.5X of
reading ?0.3% of range). For example, a display reading of 1.000 volts
ac "ill have an uncertainty L.008 volts over a frequency range from
20 HZ to 10kHz. An additional reading error may result if the source
resistance is relatively high. The input impedance of the Model 168
is frequency dependent. For example, with an input resistance of 9
megohms and shunt capacitange of less than 90 picofarads, the effoc-
tive input impedance at 60 HZ is approximately 8.23 megohms. The im-
pedance at other frequencies may be determined by the following re-
lationship.
Zin = Rin
1 + (2nf R&)Z
where 2.1~ = effective input impedance
Rin = 9 x lob ohms
sin < 90 x lo-6 farads
= frequency in HZ
I 0374 17
MODEL 168
Source loading can be determined by the following relationship:
% error = 100 x z,
% + Zi"
4. How to Determine Maximum Allowable Injxt. The maximum allowable
voltage input is 1200 volts dc + ac peak. The Model 168 displays ac
voltages to 499 volts rms. Beyond 499 "olfs the display reads properly
and blinks to indicare an overrange condifion.
NOTE
The Model 168 blocks dc signals at the input as a result of the
capacitive coupled input. Howe"er, a large dc si@,al superimposed
on an ac level could cause damage if the inpuf exceeds 1200 "olcs
dc + peak ac.
C. "oltaae Measurements using Model 1682. The Model 168 may be used
with the accessory Model 1682 RF Probe to permit ac voltage measurements
from 100 kHz to 100 EMz. The transfer accuracy of the 1682 is t5%, cal-
ibrated in rms of a sine wave. Input impedance is 4 megohms shunted by
2pF. Maximum allowable input is 30" rms AC, 200" DC. To use the 1682
with rhe Model 168, connect the probe to HI and LO. Select DC voltage
function. Voltage is direct reading in "olts ac rrxs.
The Model 1682 is designed for use with a voltmeter having an
input resistance of 10 megohms ilO%.
I 18 0374
MODEL 168 OPERATING INSTR"CTIONS
3-6. HO" TO MEASURE CURRENT. The Model 168 me=="ree ac and dc current
in four ranges; O.lmA, ImA, O.lA, and 1A. The displayed current is direct
reading in terms of milliamperes ("!A) or amperes (A) (depending on mode
selected) with decimal point located ="tam=tic=lly.
a. DC current. The Model 168 me=="ree dc current from .OOOl mA to 2 mA
end frbm .OOOlA to 1A. If the Polarity at the "I terminal is negative,
the Model 168 display indicate= a (-) minus sign. If the minus sign is
off, a positive current is implied. A lighted "DC" is displayed when the
dc function is selected. Current may be selected for direct reading in
terme of milliamperes (mA) or amperes (A) depending on the magnitude of
current measured. When the ampere= (A) mode is selected, ehe Model 168
measures current from 0.0001 ampere to 1 ampere. (The display will Per-
mit a reading up to 1.999 amperes or until the 2A fuse blows.) When the
milliampere (mA) mode is selected, the Model 168 measures current from
0.001 milli=mpere to 2 mi11*=nlperes. (The display will permit a reed-
ing up to 9.99 IDA or until the lOmA fuse blows.)
1. How to Measure DC Current. Select the dc function by depressing
the "DC Pushbutton. First select "A" mode. The "mA" and "A" pushbuteona
are "push-push" type, such that one pushbutton m"se be released before
the other can be selected. Connect the input signal between HI and LO
terminals. Observe the displayed reading, the polarity sign (for nega-
tive inputs), and decimal point position. The lighted "DC" indicates
that the dc function ha= bee" selected. The lighted ",,!A" or "A" indi-
cates that the current mode has been selected.
2. HO" to Select Ranges. The Model 168 automatically defermine= the
sensitivity for either %A" or "A" modes. If the current to be measured
is between 0.0001 mA and 2 mA, select the %A" mode. If the current to
be measured is between 0.0001 A and lA, select the "A" mode. For either
%A" or "A" modes, the Model 168 has two sensitivities, 0.1000 and 1.000
full range. On the most sensitive range, the minimum reading is .OOOO
while the maximum reading ie .1999. When the input exceeds .1999, the
Model 168 a"tom=tically "p-ranges to the higher range and decimal Point
location is changed appropriately. Down-ranging occurs when the input
level is reduced, causing a dieplayed reading to be less than 1-9-O on
any range. For example, if =n input signal is reduced from 0.190 to
0.189, the display will change from .190 to .1890. However, if the sig-
nal is increased from .1899 to .1900, the Model 168 will not "prange,
but remain on the 0.1000 range.
0374 19
OPmATING 1NSTR"CTIONS MODEL 168
FRONT PANEL
CURRENT RANGE FUSES
FIGURE 11. Locafion of Current Range Fuses.
20 0374
3. HOW to Determine Accuracy. The accuracy of the Model lb8 is t(O.3%
of reading + 0.1% of range). For example, a display of 1.000 ampere will
have an uncertainty of +.004 ampere. I" rhe ?,,A" mode, the Model 168
uses a 1000 ohm shunt resistor. In the "A" mode, the Model 168 uses a
1 ohm shunt resistor. An additional reading error shauid be considered
if the so"rce resistance is not greater then 1000 times the shunt re-
sistor. For example, in the "mA" mode, a source resistance of 100 kil-
ohm would result in a loading error of approximately 0.1% of reading.
Loading error for other eource resistances can be determined by the
following relationship:
% error - 100 X R, (mA mode)
R, + 1000
where R, = eource resistance
or % error =loo X %( A mode)
R6
4. Ho" to Determine Maximum Allowable Input. The Model 168 uses
separate fuses for each current mode. The ?,,A" mode ie protected by
a 10 milliampere fuse. The "A" function ie protected by a 2 ampere
fuse.
a) HOW to Check the Current Range Fusee. To determine the con-
dition of fuses for "IDA" and "A" modes, select the "Lo OHMS" resis-
tance mode. Since the shunt resistors are connected between HI end
LO terminals, the resistance including fuse can be meaeured directly
an the Model 168. For rhe "mA" mode, select the ?,,A" pushbutton end
observe the display. The display should reed approximately 1.180
kn to indicate a "good fuse". A flashing display indicates a "blown"
fuse. For the "A" mode, select the "A" pushbutton and observe the
display. The display should read approximaeely .0012 kn to indicate
a "good" fuse. A flashing display indicates a "blown" fuse.
b) How to Replace the Current Range Fuaee. The current range fuses
F103 and F104 are located on the printed circuit board. The fuses are
accessible by removing the fop cover and internal shield. TO replace
the fuses, turn the instrument over 80 that the bottom cover faces up.
Use a screwdriver to remove the four slotted screwe on the bottom
cover as shown in Figure 4. After the ecre"s ere removed taking cere
CO hold the top and bottom covers together, turn o"er the inetrumen~
so that the top cover faces up. Remove the top cover to gain access
to rhe printed circuit board. If the Model 1688 Rechargeable Battery
OPeRATING INSTRUCTIONS MODEL 168
Pack is installed, lift off the battery pack from the stand-offs cod
set to the left of the instrument. The fuses for current ranges are
located on the prineed circuit board near ehe front right side under
the shield. The amplifier shield must be removed to gain access to
rhe fuses. The shield.&? fastened to rhe circuit board by means of
a single Phillips head screw. Before the ecrew can be removed, the
front panel most be lifted out of, the way. Gently lift the front
panel and pull forward until panel is clear of puehbuttoos. Locate
and remove the Phillips head screw which holds the shield. The metal
shield is held by clips to fhe printed circuit board and must be pried
UP. Replace fuses as necessary. Fuse types are as follows:
FlO3: O.OlA, 25OV, type 8AC, fast acting (%A" mode)
~104: 2.4, 250", type 8AG, fast acting ("A" mode)
After fuses are replaced, re-assemble shield, spring, washer, ana
Phillips Screw as shown in Figure 11. Replace front panel. Replace
Model 1688 battery peck on stand-offs. Replace top cover. Take care
fo hold top and bortom covers together and turn over instrument so
that bottom cover faces up. Replace Screw to complete re-assembly
of chassis.
b. AC Current. The Model 168 measures ac c"rrent from .OOOlmA to 2mA
and from .OOOlA to 1A. A lighted "AC" is displayed when the ac function
is selected. Current may be selected for direct reading in cerme of
milliamperes (nut) or amperes (A) depending oo the magnitude of current
?a be measured. When the amperes (A) made is selected, the Model 168
measures currenf from .OOOl ampere to 1 ampere. (The display will permit
a reading up to 1.999 amperes or until the 2 ampere fuse blows.) When
the milliampere (mA) mode is selected, the Model 168 neasures current
from .OOOl milliampere to 2 milliamperes. (The display will permit a
reading up to 9.99 mA or until the 10 m.4 fuse bl.ows.)
1. How to Measure AC Current. Select the ac function by depressing
the "AC" pushbutton. First select "A" mode. The "mA" and "A" push-
buttons are "push-push" type, such that one pushbutton must be released
before the other can be selected. Connect the input signal between HI
and LO terminals. Observe the display4 reading, and decimal point
position. The lighted "AC" indicates that the ac fuoctioo has been
selected. The li.ghrcd "ti" or "A" indicates that the currant mode has
been selected.
22 0374
MODEL 168 OPERATING INSTRUCTIONS
2. ROW to Select Ranges. The Model 168 automatically determines the
sensitivity for either "IDA" or "A" modes. If the current to be measured
is between O.OOOlmA and 2mA, select the "m," mode. If the current to
be measured is between O.OOOlA and lA, select the "A" mode. For either
""A" or "A" modes, the Model 168 has two sensitiviries, 0.1000 and 1.000
full range. on the most SenSitiVe range, the minimum reading is .OOOO
while the maximum reading is .1999. When the input exceeds .1999, rhe
Model 168 automatically up-ranges to the higher range and decimal point
location is changed appropriately.
3. How to Determine Accuracy. The accui-acy of the Model 168 is k(l%
of reading + 0.3% of range). For example, a display of 1.000 ampere
will have an uncertainty of t.013 amperes (over a frequency range from
30 RZ to 5lcHz). In the YnA" mode, the Model 168 uses a 1000 ohm shunr re-
sister. An additional reading error should be considered if the source
resistance is not greater than 1000 times the shunt resistor. For example,
in the "ti." mode, a source resistance of 100 kiloh would result in a
loading error of approximately 0.1% of reading. Loading error for other
source resistances can be determined by the followinS relationship:
% error = 100 X R, (",A mode)
R, + 1000
4. Uaw to Determine Maximum Allowable Input. The Model 168 uses
separate fuses for each current mode. The "mA" mode is protected by
a 10 milliampere fuse. The "A" function is protected by a 2 ampere fuse.
a) HOW to Check the Current Range Fuses. TO determine the con-
dition of fuses for %A" and "A" modes, select the "LO OHMS" resis-
tance mode. Since the shunt resistors are connected between HI and
LO terminals, the resistance including fuse can be measured directly
on the Model 168. For the "mA" mode, select the ","A" pushbutton and
observe the display. The display should read approximately 1.180
kQ to indicate a "good. fuse". A flashing display indicates a "blown"
fuse. For the "A" mode, select the "A" pushbutton and observe the
display. The display should read approximately .0012 kn to indicate
a "good" iuse. A flashing display indicates a "blown" fuse.
0374 23
TABLE 3-5.
s,,mary of Shunt Resistors in Current Modes
Voleage Drop zlCTO88
Range Shunt Resistor Shunt Resistor
O.hA Ikn 0.1"
1mA lki? 1"
0.1 A In 0.1"
1.4 10 1"
*I" series with fuses F103 ("A) or F104 (A).
C. current Measurements to 50 Amperes. Current measurement capability
of the Model 168 may be extended to 50 amperes through the "se of acces-
sory Model 1651 50-Ampere Shunt. The Model 1651 permits 4-terminal co"-
nections co minimize measurement error due to lead resistance.
1. DC Measurements. Select DC voltage function. Connecf the volt-
age sensing leads of the Model 1651 to the Model 168 input terminals.
connect separate current leads (not furnished) between the source and
the large hex-head bolts on the Model 1651. The current leads should
be rated for c"rre"ts up to 50 amperes (as well as connections to the
1651). The shunt resistance is 0.001 ohm which results in a voltage
drop of 0.050 volts maximum at 50 amperes. Power dissipated in the
shunt is 2.5 watts,at 50 amperes.
2. AC Measurements. select *c voltage function. connect the volt-
age sensing leads of the Model 1651 to the Model 168 input terminals.
Connect separate current leads (not furnished) between the source and
the large hex-head bolts on the Model 1651. The current leads should
be rated for currents up eo 50 amperes (as well as connections to the
1651). The shunt resistance is 0.001 ohm which results in a voltage
drop of 0.050 volts maximum at 50 amperes. Power dissipated in the
Shunt is 2.5 watts at 50 amperes.
24 0374
VOLTAGE
TERMlNALS
I \
-\/
CURRENT
TERMINALS
CURRENT
LEADS NOT SUPPLIED
FIGURE 12. current Measurements using Model 1651.
25
0374
The largest resistance displayed in the 1.0 OHMS mode is 1.999 meg-
ohms. Beyond 1.999 megohms, the display flashes co indicate an over-
range condition although the reading will be displayed up to 2.017
megohms. Above 2.017 megohms, the display will not change. In the
ii1 ONbE mode, the maximum reading is 19.99 megohms. Beyond 19.99
me&ms, the display flashes to indicate an overrange condition ai,-
though the reading will be dLsplayed up to 20.17 megohms. above
20.17 megohms, rhe display will not change.
26 0374
Care should be taken when making resistance measurements in circuits
which may have voltages on capacitors etc. or where line voltage is
present. Although the Model 168 is fully protected against accident-
al voltages up to 25OV rms in OHMS. if higher voltages are applied
damage may occur.
d. HOW t" Determine Accuracy. The accuracy of the Mr,del 168 is f(O.2%
of reading + 0.1% of range) in HI OHMS. For example, a display reading of
1.000 kilohms will have a" uncertainty of f0.003 kilohma.
e. How to Determine the C"rrene For Each Range. The voltage developed
acro88 the resistance is directly proportional to the current applied.
For instance, a reading of 1.000 kilobms corresponds to a voltage develap-
ed of 0.900 volts in HI OHMS. The ~"rrent applied by the Model 168 is de-
termined by dividing the voltage by the resistance being measured. In the
previous example, the current is equal t" 0.9OOV i 103n - 0.9 mA. The teec
current for each range is given in Tables 3-6 and 3-7.
TABLE 3-6. TABLE 3-7.
Test Current in LO OHMSMode Test current in HI OHMS Mode
.-
Range Test Current
O.lk0 0.9*
lkfl 0.09m.4
lOkR 0.009m.4
0.lMn 0.9"A
1m 0.09"A
f. How to Test Semiconducfor Diodes and Transistors. The Model 168 can
be used to test diodes and transistors t" determine the condition of the
device. For semiconductor diodes, the voltage applied must be sufficient
to cause conduction in the forward direction. The "HI OHMS" mode of the
Model 168 provides a voltage up t" 2 volts at a current of 1 milliampere,
which is sufficient to cauee conduction. Since the "LO" terminal is poai-
rive With respect to the "HI" terminal far all resistance measurements,
connections should be made aa eh"wn in Figure 13 t" cause forward conduc-
eion of diodes.
0374 27
NOTE
me silicon diode test should be performed using the 1o"esC resistance
range of HI OHMS, (2 kilohms maximum reading) since the current used
an the higher ranges becomes small. when rhe input terminals Of the
Model 168 are open, the instrument automatically ranges to the high-
est range. TO perform the diode test, a short m"Se first be applied
to the input terminal*, causing the Model 168 to downrange to the
lowest range. An easy way to accamplish this measurement is to de-
press the "A" pushbutton (which "ill callse the Model 168 to down-
range to the loweet resistance). men connect the diode and release
the "A" pushbutton. Observe the displayed reading on the Model 168.
A reading less rtmn 1000 ohms (for silicon semiconductors) indicates
that the diode is conducting. If the diode is faulty or connected
in TeYerse, the display will flash O.lnm which indicates that cLie
resistance is greater than 20.17MR. The I.0 terminal is positive so
that anode must be connected to LO.
TO determine the semiconductor type (NPN or PNP) of a transiscar, a meas-
urement between base (B) and emirter(E) is required. First, identify the
leads of the transistor by comparing the device to the appropriate tran-
sisror configuration as shown in manufacturer's data sheets.
Nexf, measure the resistance between hasa (9) and emltfer(E). If the tran-
sistor is a,, Nl'N type, the "LO" terminal of the Model 168 should be connected
to the base (B) to cause the base-emitter junction to conduct. A properly
conducting junction "ill be leas than 1000 ohms for all NPN transistor.
If the transistor is PNP type, the "LO" terminals of the Model 168 should
be connected to the emitter (E) to cause the base-emitter junction to cm-
duct. A properly conducting junction will be less than 1000 ohms for a
PNP eransistor. TO determine the condition of a transistor, a measurement
of base-emitter and base-collector junctions is required. Table 3-8 des-
cribs the conditions which will determine if an NPN type transisror is
faulty. Table 3-9 describes the conditions which will determine if a PNP
transistor is faulty.
28 0374
I MODEL 168 OPERATING TNSTRUCTTONS
LO
+
EMITTER-TO-BASE
CONDUCTION CONDUCTION
-
HI i-) LO (+I
t t
HI- B
CATHODE ANODE
0374 29
:onnection Connection (HI OHMS)
CO LO TO HI Condirions Which Indicate
Terminal Terminal A Normal 01 Faulty Transistor.
Base Emitter A reading of approx. 700 ohms indicates a
normal junction. A reading greater than
1000 ohms indicates a faulty junction.
Emitrer Base A reading greater thatI 20.17MR indicates
a normal junction. A reading less than
20.17Mn indicates a faulty junction.
Collector Base A reading greater than 20.170 indicates
a normal junction. A reading less than
20.17MR indicates a faulty junction.
Base Collector A reading of approx. 700 ohms indicates a
normal junction. A reading greater than
1000 ohms indicates a faulfy juncrion.
TABLE 3-9.
Connection (HI OHMS)
to "I Conditions Which Indicate
Terminal A Normal or Faulfy Transistor.
Base A reading of approx. 700 ohms indicates a
normal junction. A reading greater than
1000 ohms indicates a faulty junction.
Emitter A reading greater than 20.17MCI indicates
a liornml junction. A reading less than
20.17MR indicates a faulty ,unction.
Base ea11ecror A readhg greater than 20.17.W lndica~es
* normal junction. A reading less than
20.17MO indicates a faulty junction.
Collector BaSe A reading of approx. 700 Dhms indicates a
normal junction. A reading greater than
1000 ohms indicates a faulty junction.
30 0374
MODEL 168 THEOKY OF OPEKATION
SECTION 4. THEORY OF OPERATION
4-1. GENERAL. This section coniains information to describr the Elodci I68
circuit operatim.
a. Class-epoxy printed circuit boards are used for all circuitry. 'The
analog and digital circuitry is located on mother board, PC-346. 'rile clip,-
ital display circuitry is located on display board, ~~-347.
b. Compactness and high reliability are provided rhrough ~hc ust: of A
digital LSI, a completely solid-state LED displ~ay, thick-film resisror
networks, and linear integrated circuits.
All circuit designations refer to components shown on schematics
26088E and 26089E located on pages 85 and 86.
4-2. ANALOG CIRCUITRY
*. First stage Amplifier. This amplifier is * FET input i"Legratcd
circuit (QA103) connected as an inverting amplifier. Gain for the ml,-
lifter is set by automatically switching resistors with JFCT'S into the
feedback of the amplifier. Potentiometer R135 is an input zero adjust-
ment on the front panel.
b. Second Sfage Amplifier. This amplifier provides additional gain f"r
rhe most sensitive ranges. Integrated circuit 4.4104 is connected as an
inverting amplifier with a gain of X1 or X10. Potentiometer R139 (-IV AD.J.)
is a gain adjustment for x10 gain. Potentiometer R115 is a zero adjusfment
for Xl gain. Porentiometer R137 is a zero adluetment for Xl0 gain.
TABLE 4-l.
DC Cain Switching (UP-Ranging)
Iboge First stage Gain Second Stage Gain Overall Gain
0.9 10 9.0
0.09 10 0.9
0.009 10 0.09
0.009 1 0.009
0.0009 1 0.0009
0374 31
0374
I MODEL 168 THEORY OF OPERnTTON
rlG"Re 15. Full wave Rectifier.
0374
33
100" 0.009 0.009
10" 0.09 ".09
I" 0.09 0.9
0.1" 0.9 9
MODEL 168 THEORY OF OPERATION
35
I THEORY OF OPERATION MODEL 168
TABLE 4-3.
RangeRI RF Gain
0.1"
1 ox- IOV
10M
IOM
9M
900K
0.90
0.09
10 o* 1oov 10M 90K 0.009
1000" 10M 9K 0.0009
0.1" ZK 20K 10
1" 2K 20K 10
1ov 20K or 2K 20K 1 c!r 10
1oov 20K 20K 1
1000" 20K 20K 1
4-4. AC "OLTAG~ OPERATION.
a. Gain. In the ac mode, the input amplifier has a unity gain on the
0.1 volt range decreasing to 0.001 on the 1000 volt range. The gain far
each range is determined by the feedback components as shown in Table 4-5.
0.1" 55 pF 9M 9M 55 pF I
1 or 10" 55 pF 9M 900K 550 pF 0.1
10 or 100" 55 pF 9M 90K 0.0055 !lF 0.01
1000" 55 pF 9M 9K 0.055 UF .OOl
6. Frequency Response. me frequency response is from 20HZ to 1OkHZ on
1, 10, and 100" ranges, 20Hz to 5kHz on .lV and SOOV ranges. The ac ranges
are calibrvted eirraugtl tile use of three trinmling capacitors C108, c110, and
c113.
I 36 0374
I MODEL 168 THEORY OF OPERATION
I THEORY OF OPERATION MODEL 168
4-5. OHMS OPERATIQN. n, the OHMS mode, the input terminals ("HI" and
"LO") are connected in the feedback of ~~103 so as to reduce the slowing
effects at cable capacitances. When the OHMS mode is selected, constant
current in decade steps is applied between the input terminals with the
LO terminal posifive. The OHMS reference is composed of integrated
circuit &WI6 and range resistors RNLOl-1, -4, -5, and -6 which are also
used on voltage modes. An addieianal resistor RIO5 is used on the Ikn
range. The test current is generated by the-.81" reference volta'@ and
the range resistor. The voltage developed across the terminals is pro-
portional to the measured resistance. For example, when a 15 megohm re-
sister is connected, the voltage developed is .81" t 9M x 15M = 1.35V.
PaCentiomeCer El22 is the adfustmenc for the 1OOkR range. This control
sets the output of QUO6 to approximately -.81 volts. Potentiomefer
R109 is the adjustment for the lkn range. Adjustments made in high ohms
mode only. Maximum open circuit voltage across the terminals ia 6 volts
in series with 9Mfl in eirher HI or LO OHMS.
TABLE 4-6.
Test Currenr in HI OHMSMod◦ Jabse Service Manual Search 2024 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo